1. Field of the Invention
The present invention relates to an image signal processing method advantageously applicable to a digital camera, image inputting unit or similar imaging apparatus of the type interpolating pixel data in virtual pixels by using the pixel data of real pixels.
2. Description of the Background Art
In a digital camera, for example, the number of photosensitive cells or pixels bidimensionally arranged on a solid-state image sensor determines the resolution of an image. Resolution increases with an increase in the number of pixels arranged in a limited area available with the image sensor. On the other hand, for a given number of photosensitive cells, the influence of noise, for example, decreases with an increase in the size and therefore sensitivity of the individual photosensitive cell, enhancing image quality. Stated another way, the size of the individual photosensitive cell decreases with an increase in the number of pixels that should be arranged in the limited area. As a result, sensitivity falls due to the decrease in the size of the individual photosensitive cell although the number of pixels and therefore resolution may increase. This limits image quality available with the image sensor.
Japanese patent laid-open publication No. 2000-184386, for example, discloses a signal processing method and a solid-state image sensor addressing the above-stated contradictory demands. Specifically, the image sensor disclosed in the above document includes photosensitive cells shifted from each other by one-half of a pixel pitch in the horizontal and vertical directions in a so-called honeycomb pattern. The honeycomb pattern allows the greatest possible number of pixels to be packed in a limited space. The individual photosensitive cell is provided with a relatively large size and therefore sufficient sensitivity for the number of pixels arranged, thereby protecting image quality from degradation. To use the resulting pixel data more effectively, pixel data are interpolated in virtual pixels where the photosensitive cells are absent, as distinguished from real pixels where the photosensitive cells are present. The real pixels and virtual pixels both are dealt with as pixels in order to further increase resolution. In addition, the frequencies of signal components of luminance data and color data are raised for thereby enhancing image quality.
As for a virtual pixel, the prior art signal processing method described above detects a correlation between pixels in each of the oblique direction and vertical and horizontal directions and then selects one direction in which the degree of correlation is highest. Subsequently, the method interpolates pixel data in the virtual pixel by using pixels positioned in the direction having the highest degree of correlation, thereby improving image quality (adaptive interpolation).
However, the prior art interpolation of virtual pixels is apt to degrade image quality. For example, when a subject includes chromatic and achromatic lines over a particular width, a difference in level occurs in the direction of length due to such a configuration of the subject. Also, when a resolution chart is shot, spurious signals intersecting the wedge-shaped portion of the chart, which is the index of resolution, appears in the resulting image. In addition, discontinuous lines appear in the image.